I think that GamerPay might be in the clear, but they require Web API keys from users. The terms [1] of those keys don't seem to allow for sharing API keys with third parties like this:
>
2. License to Steam Web API & Steam Data. Subject to these Terms of Use, you may access the Steam Web API, implement the Steam Web API in your Application, and distribute Steam Data to end users for their personal use via your Application, all in accordance with the Steam Web API documentation. This license is subject to the following restrictions: [snip]
> You agree to keep your Steam Web API key confidential, and not to share it with any third party. This license is personal to you and specific to your Application. You agree that you will be personally responsible for the use of your Steam Web API key.
When you request an API key from Steam they ask you to include the domain of your application. GamerPay instructs users to just put their own username in that field.
> 2. License to Steam Web API & Steam Data. Subject to these Terms of Use, you may access the Steam Web API, implement the Steam Web API in your Application, and distribute Steam Data to end users for their personal use via your Application, all in accordance with the Steam Web API documentation. This license is subject to the following restrictions: [snip]
> You agree to keep your Steam Web API key confidential, and not to share it with any third party. This license is personal to you and specific to your Application. You agree that you will be personally responsible for the use of your Steam Web API key.
When you request an API key from Steam they ask you to include the domain of your application. GamerPay instructs users to just put their own username in that field.
[1]: https://steamcommunity.com/dev/apiterms